1. Field of the Invention
The present invention relates generally to sensors, such as proximity sensors or photoelectric devices, which are externally programmable to alter an operating characteristic of the sensor and, more particularly, to sensing devices of this type which utilize externally located magnetic components or magnets to alter an operationally characteristic response of an electronic circuit disposed within the sensor body in order to change the response of the sensor to a predetermined stimulus.
2. Description of the Prior Art
Many different types of sensing devices are known to those skilled in the art. For example, certain types of proximity sensors operate to detect magnetic material within a zone proximate a portion of the sensor. In other systems, photoelectric devices are used to detect the presence or absence of an object within a zone through which a light beam is directed.
U.S. Pat. No. 4,433,309, which issued to Hermle et al on Feb. 21, 1984, describes a proximity switch that is provided with a built-in test circuit. It illustrates a circuit for an inductive proximity switch that comprises a built-in testing capacity for monitoring moveable elements in which an oscillator has an inductive control element. The impedance of the inductive control element varies in response to approaching elements with a built-in electrical tester for monitoring the correct functioning of the proximity switch, wherein the tester has an electrically switchable damping circuit for damping the oscillator for test purposes.
U.S. Pat. No. 4,800,238, which issued to Gesvent on Jan. 24, 1989, describes an electronic device, such as a sensor, which has a housing with a pivoted adjustment access sealing cover in which the cover is hinged by means of a laterally oriented pivot portion held captive in recesses cooperatively formed by the housing and a retainer. The housing and the retainer are formed of the same ultrasonically weldable material and are ultrasonically welded together. The cover is formed of a material different from the material of the housing and retainer to resist ultrasonic welding and remain free for pivotal movement.
U.S. Pat. No. 4,334,204, which issued to Buchanan on Jun. 8, 1982, discloses a proximity switch assembly in which the self contained assembly is of the type which indicates the status of a moveable mechanical structure, such as a door, being in proper position for latching. It includes a proximity sensor mounted to a base plate. Also mounted to the base plate is a plunger mechanism which carries a target plate. Upon the striker surface of the plunger engaging an external object, the target plate moves into proximity with the sensor and thereby produces a status signal.
U.S. Pat. No. 4,796,966, which issued to Kovaleski et al on Jan. 10, 1989, describes a magnetically activated fiber optic switch. The switch is provided for regulating light transmission comprising an encasing movement bodies surrounding a paramagnetic shaft, a section of which protrudes therefrom. A spring member surrounds the shaft protruding from the encasing movement body. At the end of the shaft opposite that of the protruding section is a light intercepting surface that is alienable with an end surface of at least one optical fiber. When a magnet outside the encasing body approaches sufficiently close to the shaft, magnetic forces attract the shaft overcoming the restraining spring and cause the light intercepting surface to move out of alignment with ends of the optical fiber and thereby makes or breaks a light circuit.
U.S. Pat. No. 4,742,217, which issued to Wilwerding on May 3, 1988, discloses a projection alignment and focusing aide. The device is used to assure that a detector array receives a properly focused image from a predetermined area. The image is of a remote object. The device provides an elongated radiation source that is located at an optical position which is indicative of the location of the detector array. It uses a partly reflective mirror in the path between the object and the array for the purpose of projecting radiation from the source to the object so that the proper focus condition and the desired area may be adjusted by focusing and positioning the image of the radiation source on the object.
Many different types of sensors, such as those described above, are used to detect the presence or absence of an object and, in some situations, to detect the distance of an object from a predefined location. For each particular application of a sensor, several operational characteristics of the sensor must be determined prior to installing the device in conjunction with associated equipment. For example, it must be initially decided whether the circuitry of the sensor, whether the sensor is a proximity sensor or a photoelectric device, is to operate as a normally open or normally closed switch. Furthermore, the operational range of the sensor must be determined. Additionally, it must be decided whether the circuitry of the sensor should sink or source electric current. Other initial parameters can relate to diagnostic capabilities, short circuit protection and TTL capability.
One approach to satisfying these requirements is to provide many different types of sensors. In other words, the user of the sensors could stock a predetermined number of sensors that operate as normally open switches and another predetermined number of sensors that operate as normally closed switches. Similarly, both extended range sensors and standard range sensors could be maintained in inventory and sensors that sink current would be stored along with sensors that source current. The requirement to maintain an inventory stock of many different types of sensors that are each basically the same in operation, but exhibit different operational characters in response to a stimulus, is both expensive and wasteful.
It is known to those skilled in the art that a sensor can be provided with selective switching that changes the response characteristics of the sensor to preselected stimuli. However, this capability typically requires the provision of switches connected to the sensors that permit the operator to selectively change the operationally characteristic response during initial installation in conjunction with associated equipment. The inclusion of switches to permit the selectability of operational characteristics unfortunately creates an additional problem. Switches of this type require that a breach be provided in the housing of the sensor. Any opening of this type in the housing makes the sensor vulnerable to the passage of deleterious fluids through the switch openings to the internal portion of the sensor where its circuitry is contained. This vulnerability is particularly disadvantageous in applications where the sensor is required to withstand a periodic washdown wherein high pressure fluids are directed against the outer surface of the sensor. If selectable switching is provided that requires a breach in the outer housing of the sensor, the high pressure washdown will cause fluids to pass into the internal portion of the sensor housing and have a damaging effect on the operational integrity of the internal circuitry.
If would therefore be significantly beneficial if a sensor could be provided which comprises a waterproof housing without any openings in it while also providing the capability of changing the operational characteristics of the internal circuitry to change the response of the circuitry to predetermined stimuli.